The present invention relates to electronic circuits, and more particularly to an open-drain output buffer adapted to operate at relatively high voltages.
To realize manufacturing and economical leverages, topological geometries of semiconductor devices have been continually scaled downward across successive product generations. Supply voltages for semiconductors have correspondingly scaled downward, at least in part, to maintain consistent working voltages across materials, such as gate oxides. Historically a 0.35 micron (μm) technology has incorporated a 3.3 Volt (V) supply voltage and correspondingly, 0.18 μm and 0.13 μm technology generations have used 1.8 V and 1.2 V supplies, respectively. Maintaining consistent maximal operating voltages is necessary to avoid over-voltage conditions across electrical terminals that expose corresponding materials to electric field magnitudes that would cause material breakdown and device failure. The challenge of maintaining operating voltages within electrical limits of material properties comes at the input and output terminals of the semiconductor device. The input and output terminals are where an operating voltage region of a first device interacts with the voltage region of a second device. The device most challenged is the one operating in a lower voltage region. During electrical switching between the two operating voltage regions, the first device, operating at the lower voltage, experiences voltage from the second voltage region that may exceed operational voltage limits of the first device. During voltage excursions to the upper logic levels of the second device, over-voltage conditions in the first device are likely to cause exposed materials to fail.
Output buffers with open-drain pull-down transistors are typically used for attachment to common buses with other transistors (usually in another package). A single voltage supply point, perhaps with a pull-up resistor to a power source, provides the highest logic level required by any switching transistor on the bus. Output buffers with open-drain pull-down transistors are commonly fabricated in complementary metal oxide semiconductor (CMOS) processes. As an output terminal of an open-drain CMOS buffer turns off, pull-down transistors are switched off and buffer terminals remain in electrical connection with the output pad. An open-drain buffer of the first transistor (as above) experiences a high voltage level corresponding to an upper logic level voltage coming from the second transistor. The magnitude of the high logic-level of the second transistor, when applied to terminals of the first transistor may provide voltages that exceed the operating voltages and maximum sustainable voltages for particular materials in the first transistor. To avoid damage, the pull-down transistors have to be maintained in a semiconductor well provided with a voltage equal to the voltage provided by the second transistor and no gate oxide of a switching transistor may be exposed to a voltage causing failure of the gate. To avoid material breakdown, transistors exposed to elevated external voltages have been placed within a well provided with voltage near the switching voltage levels.
Typically, designers have found ways of providing a biasing voltage level to a substrate well encompassing a given switching transistor exposed to a relatively higher voltage region. Presuming that no explicit connection to the higher voltage region exists for the first transistor, a designer has been faced with utilizing some means of providing a path from the external voltage source to provide biasing to a well-region isolated from the well-regions operating at the native voltage-region level. Often the isolated or floating well-region is coupled to the output pad by a coupling transistor having a conductance characteristic provided and triggered by the elevated external voltage level. The coupling transistor provides an electrical path to the floating well providing the external voltage level as a well bias. This technique has been limited to a relative voltage level of about two times the operating voltage (VDD) of the first transistor. In order to provide a broader possible range of interface voltage interactions between semiconductor transistors, a means of allowing a greater range of disparity between voltage regions being switched to-and-from would be desirable. It would also be desirable to have a way of incorporating the voltage level of the external region and yet, still incorporate the floating well principle, and at the same time allow continued use of less expensive process technologies for the implementation of the interface transistor.